1. Field of the Invention
This invention relates generally to methods for configuring aircraft, and more particularly to design methods and configurations for supersonic aircraft.
2. Background
Current supersonic aircraft designs provide passengers and cargo with reduced flight times, but at the cost of the noise produced by sonic booms. Due to adverse public perception of the noise associated with sonic booms, civil regulations currently prohibit overland supersonic flights in the continental United States. As a result, successful business and commercial aircraft development has generally been limited to subsonic designs. A variety of supersonic military aircraft designs are operationally employed, however, the scope of military supersonic flight operations is sometimes limited due to sonic boom noise.
The theory of sonic boom reduction has been in existence since the 1960s. However, no supersonic aircraft that incorporates sonic boom reducing design features has ever entered production or operational use. Many design studies have been performed, but few have led to promising designs. Implementing a constrained sonic boom signature imposes an exact requirement on the distribution of a quantity called “equivalent area” along the lengthwise axis of the vehicle. Equivalent area at a given location is the sum of a term that is related to the local cross sectional area at that location, plus a term that is proportional to the cumulative lift between the nose of the aircraft and the given location. Thus, the equivalent area distribution involves a combination of the cross sectional area distribution and the lift distribution.
Prior attempts to design passenger aircraft with reduced sonic boom have typically used cross-sectional area only, at least as far aft as the beginning of the passenger cabin, to provide the required equivalent area distribution. With the lift distribution beginning aft of that point, lift must then be built up fairly rapidly in order to provide the center of lift at the center of gravity. The tradeoff between lift and cross section then produces a pinched section near the middle of the vehicle. This pinching is known as “area-ruling” and it is common even on supersonic vehicles which are not designed for reduced sonic boom. However, designing to a sonic boom requirement tends to aggravate the pinching if conventional design approaches are followed.
Such pinched-fuselage designs suffer from several shortcomings. For example, a small fuselage cross section is undesirable near the middle of the fuselage, where maximum bending strength is required and where the main landing gear is typically stowed. A pinched midsection also makes it more difficult to balance an aircraft configuration at the center of lift because the lift distribution tends to be too far aft relative to the location of the payload. However, moving the center of lift forward tends to further aggravate the pinching of the fuselage. Furthermore, with such pinched-fuselage designs, useful volume for payload and fuel tends to lie in the forward and aft extremities of the aircraft, respectively. This widely dispersed mass distribution tends to lead to a high mass moment of inertia about the pitch axis, high structural loads and large variations in the location of the center of gravity. In addition, the relative large forward fuselage cross section tends to be larger than optimum for wave drag.